I need to monitor the flow of a few pumps on my boat so i can accurately measure the amount of water (weight) that is being pumped into my ballast. i found these great sensors:http://www.seeedstudio.com/depot/g12-water-flow-sensor-p-635.html?cPath=144_151

but they are 1/2" threads. my pumps are 1" and the restriction is unacceptable since speed is an asset with my set up. also they only flow 30 l/min which is about 480 gph. where as my pumps will flow 720 gph.

do you guys have any suggestions on a practical (read inexpensive) way to measure flow rate? i was thinking about using a drill pump in reverse (having the pump spin the drill spindle. and then just hooking that up to a hall sensor or rotary encoder. but i'm not sure about how much work it'd take to spin that thing.

I had thought about the two flow meters but 2 would flow around 480 gph and my pump pushes out 720 gph. i'd have to get 3 per pump and that would (15 flow meters)the tanks are actually soft and they collapse. i did think about making a small 'hard tank' out of 3" pvc that i could pipe the outlet port of the bag into it. then use some sort of height/distance sensor. but that wouldn't work for the bow ballast since there will be two of them seperated (one would go under the seats and a second would go in the ski locker about 1 ft below).

i do have a vent port on the bags. but since they are soft ballast i dont think it would be as reliable to measure the air.

Watermeters.com offers nice bronze units with 1pulse/gallon, see datasheet here. They are not cheap, but any water meter with any sort of accuracy at this port size (i.e. low restriction) is expensive. That's still not the world's best resolution but better than nothing (i.e. ~8lb / pulse).

Depending on how sensitive your pumps are to dead-heading, the flow restriction is not an issue - review the pump curve. Thus, if speed is not an issue (i.e. you're OK with waiting 2x+ longer than usual) you may be able to use those cheapie sensors you referenced. I wouldn't use such hardware below the water line, however.

A less expensive option (open-loop) is to put a current switch on the pump. Measure the length that the pump has been running, multiply by a known flow rate (and that has to be verified with the local head conditions!) and voila. Current switches are cheap, Current Magnetics is one company that sells them.

You can also use a flowmeter in parallel with a pipe. In theory the flow will be proportional to the diameter of the two pipes. In practice you have to make measurements and create a lookuptable for different flowrates.

The form (angles) of the connection of the pipes will determine how much goes through each pipe, you might need to experiment with that too as you like to have the max nr of pulses when the pump is at 100%.

Not trivial testing, but can be done (watermeasurements are preferably done on a hot day

Watermeters.com offers nice bronze units with 1pulse/gallon, see datasheet here. They are not cheap, but any water meter with any sort of accuracy at this port size (i.e. low restriction) is expensive. That's still not the world's best resolution but better than nothing (i.e. ~8lb / pulse).

Depending on how sensitive your pumps are to dead-heading, the flow restriction is not an issue - review the pump curve. Thus, if speed is not an issue (i.e. you're OK with waiting 2x+ longer than usual) you may be able to use those cheapie sensors you referenced. I wouldn't use such hardware below the water line, however.

A less expensive option (open-loop) is to put a current switch on the pump. Measure the length that the pump has been running, multiply by a known flow rate (and that has to be verified with the local head conditions!) and voila. Current switches are cheap, Current Magnetics is one company that sells them.

cheap is better for me right now. since i'll have 5 pumps and don't 'really' need this feature. it's more of a luxury. speed is a factor. it usually take 8-10 minutes to fill my ballast bags. which is about the right amount of time. by the time i get my board on and suited up. im at that threshold. waiting another 10 minutes would be annoying.

I was going to go with a timer based setup like you had mentioned (the pumps are being controlled by an arduino so i could skip the current switches). but i'm not sure how much flow is affected from the speed of the boat v. boat floating. that could greatly affect flow rate.

You can also use a flowmeter in parallel with a pipe. In theory the flow will be proportional to the diameter of the two pipes. In practice you have to make measurements and create a lookuptable for different flowrates.

The form (angles) of the connection of the pipes will determine how much goes through each pipe, you might need to experiment with that too as you like to have the max nr of pulses when the pump is at 100%.

Not trivial testing, but can be done (watermeasurements are preferably done on a hot day

i like this idea a lot. I did end up getting a cheap drill pump which i'm going to modify with a hall sensor and magnet. sort of making a cheap and high volume flow sensor. if that doesn't work. i'm giong to test this idea out.

A ratiometric approach may certainly work. The drill pump is a good idea also. You may be able to skip the hall effect sensor and use a infra-red system as well if the vanes are visible. Such a sensor might be easier to mount and more rugged.

A ratiometric approach may certainly work. The drill pump is a good idea also. You may be able to skip the hall effect sensor and use a infra-red system as well if the vanes are visible. Such a sensor might be easier to mount and more rugged.

hmm.. the vanes are behind a black abs case. i could potentially put a cylinder with a hole drilled through it on the drill shaft portion of the pump. then do the infra-red set up. are you thinking that the magnet would introduces some sort of wobble or harmonic distortion?

oh yeah, i understand not to drill the case but if i attached a cylinder to the shaft that the drill chuck clamps down on. then drill a hole into that cylinder. put the sensor on one side and the ir led on the other. it would get two pulses per revolution..

I've tried one of those sensors on the inlet to our turkey barn, on a fairly hot day, with 10,000 birds drinking like crazy I was measuring 350-450 l/hr!You could hear the little turbine in the sensor literally humming away!

From the mess these charming creatures make when they break a water line, I can tell you that it was an accurate reading!The little *$%&^'s can empty a 2000l header tank onto the barn floor in a couple of hours!

So I think the sensor can measure quite a bit more than SeeedStudio say!

I have no doubt that you can 'push' sensors of this type beyond their intended ranges. However, you do risk the possibility that the output becomes non-linear or otherwise deviates from the actual once you go outside the specifications that the manufacturer has laid down.